CAIRO — Twice in the past seven days, Egypt and the United Arab Emirates have secretly teamed to launch airstrikes against Islamic-radical militias battling for control of Tripoli, Libya, four senior U.S. officials said, in a major escalation of a regional power between the supporters and opponents of political Islam set off by Arab Spring revolts.

The United States, the officials said, was caught by surprise: Egypt and the Emirates (UAE), both close allies and military partners, acted without informing Washington or seeking its consent, leaving the Obama administration on the sidelines. Egyptian officials explicitly denied the operation to U.S. diplomats, the officials said.

The strikes are another high-risk and destabilizing salvo unleashed in a struggle for power that has broken out across the region in the aftermath of the Arab Spring revolts, pitting old-line Arab autocrats against Islamic fundamentalists.

Since the military ouster of the Muslim Brotherhood president in Egypt one year ago, the new Egyptian government, Saudi Arabia and the United Arab Emirates have formed a bloc exerting influence in countries around the region to roll back what they see as a competing threat from Islamic extremist. Arrayed against them are the Islamic political movements, including the Muslim Brotherhood, backed by friendly governments in Turkey and Qatar, that sprang forward amid the Arab Spring revolts.

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U.S. officials said the Egyptians and the Emirates had teamed up against an Islamist target inside Libya at least once before. In recent months, the officials said, teams of “special forces” operating out of Egypt but possibly composed primarily of Emirates had also destroyed an Islamist camp near the Libyan city of Derna, an extremist stronghold.

Libya is the latest, and hottest, battleground. Several officials said that U.S. diplomats were fuming about the airstrikes, believing they could further inflame the Libyan conflict at a time when the United Nations and Western powers are seeking a peaceful resolution.

“We don’t see this as constructive at all,” said one senior U.S. official.

Officials said that the government of Qatar has already provided weapons and support to the Islamic fundamentalist aligned forces inside Libya, so the new strikes represent a shift from proxy wars — where regional powers play out their agendas through local allies — to direct involvement.

The strikes have also proved counterproductive so far: The radical Islamic militias fighting for control of Tripoli successfully seized its airport the night after they were hit with the second round of strikes.

U.S. officials said Egypt had provided bases for the launch of the strikes. President Abdel-Fattah el-Sissi of Egypt and other officials have issued vigorous but carefully worded public statements denying any direct involvement inside Libya by Egyptian forces. In private, officials said, their denials had been more thorough.

The officials said that the UAE — believed to have one of the most effective air forces in the region, thanks to U.S. aid and training — provided the pilots, warplanes and aerial refueling planes necessary for the fighters to bomb Tripoli out of bases in Egypt.

The UAE has not commented directly on the strikes. But Monday an Emirati state newspaper printed a statement from Anwar Gargash, minister of state for foreign affairs, calling questions about an Emirati role “an escape” from the recent election that he suggested showed a desire for “stability” and a rejection of the Islamic extremists. The allegations about the UAE role, he said, came from a group who “wanted to use the cloak of religion to achieve its political objectives,” and “the people discovered its lies and failures.”

The UAE was once considered a sidekick to Saudi Arabia, a regional heavyweight and the dominant power among the Arab monarchies of the Persian Gulf. The Saudi rulers, who draw their own legitimacy from a puritanical understanding of Islam, have long feared the threat of other religious political movements, especially the well-organized and widespread Muslim Brotherhood.

But Western diplomats in the region say the UAE is now far more assertive and aggressive than even the Saudis about the need to eradicate Islamist movements around the region, perhaps because the Emirati rulers perceive a greater domestic threat.

The issue has caused a rare schism among the Arab monarchies of the Gulf because Qatar has taken the opposite tack. In contrast to its neighbors, it has welcomed Islamist expatriates to its capital, Doha, and supported their factions around the region, including in Libya.

The first strikes occurred before dawn a week ago, hitting positions in Tripoli controlled by Islamic radical-friendly militias, blowing up a small weapons depot and killing six people.

A second set of airstrikes took place south of the city early Saturday, hitting rocket launchers, military vehicles and a warehouse all controlled by Islamic militants.

The second strike might have been motivated by a desire to prevent an imminent capture of the Tripoli airport by political-Islam- aligned militia, many of whom are based in the city of Misurata and more tribal than Islamic fundamentalist in orientation. It had previously been held by militias based in Zintan and aligned against the Islamists. But after besieging the airport for a month, the Islamist aligned forces overtook it that night.

Anti-political-Islamic forces based in eastern Libya under the renegade former general Khalifa Hifter sought to claim responsibility, but their statements were inconsistent and the strikes were beyond their known capabilities.